Most conventional optical deflection devices involve the use of optically flat mirrors, either alone or in combination. An advantage to using this type of mirror, as opposed to crystalline or reflective lenses, is that a collimated beam of light can be redirected without significant divergence or focusing. This becomes important in applications where the deflected beam must be detectable at great distances.
A typical mirror is formed by depositing a reflective coating, such as gold, upon an optically flat substrate. These surfaces, however, can be seriously degraded, or even vaporized by a sufficiently powerful optical signal. Laser-based communications networks or weapon systems positioned in space could employ such high energy laser beams.
As an alternative to mirrors, it is possible to use transparent optical flats to deflect a beam of light. If a glass or quartz flat is oriented at an oblique angle to an incoming beam of light, a controlled deflection will result owing to the increased refractive index of the glass or quartz. Unfortunately, crystalline flats used in this way will produce little deflection, and because their refractive indices are fixed, the deflection may only be controlled by mechanically repositioning the flat.
Most electronic and electro-optic beam steering techniques are based on the use of periodic changes in a crystal's refractive index due to acousto-optic or photorefractive effects. See Sincerbox, G. T., Roosen, G., "Opto-optical Light Deflection," Applied Optics, Vol. 22, No. 5, 690 (1983). Thus, Fischer, et. al., U.S. Pat. No. 4,869,579, teaches the use of a third order nonlinear crystal such as BaTiO.sub.3, together with two incident pumping beams differing in either phase or frequency, to create a periodic variation of refractive index, or index grating, such that the beams are diffracted at a controllable angle.
Thermal effects were used by Bialkowski, U.S. Pat. No. 4,585,301, to create an optical switch within a photorefractive material. A control beam passing through the medium sets up a thermal lens or gradient. A second beam is deflected by the "lens" to a prepositioned detector. The invention does not rely on the change in refractive index at the interface of an on-linear material, nor is it limited to a localized change in the refractive index (i.e. not making use of reflection). For this reason, Bialkowski suggests the use of FREON 12 as an absorbing gas to create the thermal gradient, and argon as an inert gas for mixing.
When non-linear optical gases have been used, it has mainly been to produce phase conjugate replicas of an incident beam, typically in reflection. See Fukuda, U.S. Pat. No. 4,869,578; Wang et al., U.S. Pat. No. 4,233,571. Fukuda teaches a gas-dynamic phase-conjugated mirror, and notes that non-linear gases, as opposed to non-linear liquids or solids, can withstand very hig light intensities (&gt;2.times.10.sup.9 W/cm.sup.2). For this reason, phase conjugating mirrors have found use in laser fusion applications.
In situations where it is desirable to focus a laser beam on targets at great distances in space, it is necessary to use focusing optics with very large output apertures (up to 5 meters in diameter) and near diffraction limited performance. Crystalline lenses of this size are difficult to produce with good optical quality, and may be too heavy to boost into space. Additionally, a crystalline lens is limited to but one focal length as a consequence of its shape and refractive index.
Roberts and Honeycutt, U.S. Pat. No. 4,512,639 (1985), teaches an erectable large optic for outer space applications utilizing a gas dynamic lens of fixed focal length. Jets of gas having a refractive index, n, are used to deflect and focus a diverging laser source. As the gas disperses, the refractive index of the gas decreases so that the index varies from the outer boundary of the lens to its inner boundary. Nitrogen, helium, and oxygen are listed as possible gases for this use.
To overcome focal length limitations, Roberts and Honeycutt, U.S. Pat. No. 4,582,398 (1986), suggests the use of a large continuously focusable gas lens. Essentially a large balloon, it is fabricated from a material such as chloride/vinylidene chloride copolymer (Saran wrap), and is inflated using gas dynamic nozzles. To alter the focal length of the balloon it is mechanically deformed, making changes in deflection slow and inaccurate.
A gas zoom lens is described by McCrobie et al., U.S. Pat. No. 4,331,388 (1982) for use with a camera. A plurality of conventional lenses are positioned around a central cavity which is filled with an optically transparent gas such as FREON. By altering the cavity pressure the refractive index of the gas is changed, thereby allowing variable focal lengths. This magnification system, however, is of little use as a beam steerer or deflector. In space-based applications, for example, the crystalline optics would prove too heavy, and their fabrication too costly.
Accordingly, it is an object of the present invention to provide an optical deflection device comprising only gas-filled optics to achieve high optical quality, low attenuation, light weight construction, and the ability to withstand very intense beams of light.
It is another object of the present invention to provide an optical deflection device comprising only gas-filled optics, which allows rapid variation of a beam's direction through the use of photorefractive gases, which upon optical stimulation experience a change in refractive index and affect the path of a second beam of light.
It is still another object of the present invention to provide an optical deflection device comprising only gas-filled optics, which allows rapid variation of a beam's direction through the use of optically non-linear gases, which upon optical stimulation set up stimulated Brillouin scattering, which in turn causes acoustic waves to be generated by electrostriction. As a result, the density, and therefore the refractive index, of the non-linear medium is modified in response to the optical signal, and the path of a second optical beam may be altered.
It is also an object of the present invention to provide an optical deflection device comprising only gas-filled optics, which in addition to having means for opto-optical deflection, is provided with means to either mechanically position the gas lenses, or to alter the pressure of the gas in at least one of the lenses.